The harvesting skill bonus (ie Gathering, Foresting, etc.) is relatively minimal and is based on the appropriate skill -- if you have 600 Gathering and 450 Foresting, when harvesting lumber it looks at the 450 not the 600. Also keep in mind that at best that skill bonus just lets you roll on the "bonus" table, which has marginally higher chances of getting a rare or a 3/5 pull instead of a 1.
On a side note, double harvest chance is by far the more important stat for harvesting. If you have 50% double harvest chance, your rares per hour (or wahtevers per hour) basically increases by 50% because you're getting more free pulls. The effect of the regular harvesting skill is much smaller, as the difference between the normal and "bonus" harvesting tables.
Of course since those stats tend to come together anyway it's not too much of an issue. But if it's a choice between harvesting skill and double harvest chance, go for the double.
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